This invention relates to a method of continuously removing volatile hydrocarbons from an elastomer solution which may contain up to about 85% by weight of such hydrocarbons. More particularly, the invention relates to a method of continuously removing volatile hydrocarbons from an elastomer solution to produce an elastomer product having less than about 1.0 percent by weight of volatile hydrocarbons.
In the manufacture of synthetic rubber by polymerization in an organic diluent medium, the polymer produced must be dried or separated from the diluent before it can be used. In the past, a number of methods of accomplishing this result have been proposed. The most popular early method of dissolventizing synthetic rubber involved coagulation of the rubber in solvent solution by discharging it into water and heating the water so as to steam-distill off diluent and precipitate the polymer into crumbs. The crumb is then dried in a conventional manner, such as by means of a de-watering press and a hot air oven drier.
Alternative methods for removing the water from rubber crumb have employed a screw extruder containing vents along the barrel, through which water vapor passes as the rubber is kneaded and masticated in the extruder. A method and apparatus of this type are shown in U.S. Pat. No. 3,225,453, wherein wet rubber crumb is dried essentially completely by passing it through a vented extruder. The extruder performs mechanical work on the rubber to heat it and to express water from it, and the water in liquid and vapor form is expressed through drainage openings extending the length of the extruder. This process has been found effective in removing water from rubber, since the tearing action of the extruder opens pockets of water contained within the crumb to release moisture.
More recent techniques for drying solution-polymerized elastomers have attempted to avoid the step of coagulation in water. In these newer methods, the problem is one of removal of organic diluent and unreacted monomers only, presenting a new and different system for polymer drying. Since the elastomer, diluent, and monomers are mutually miscible throughout all proportions, in most cases, the situation differs considerably from the process described in the reference patent wherein all the volatile material (water) is immiscible with the elastomer. Examples of such recent techniques for drying solution-polymerized elastomers are U.S. Pat. Nos. 3,683,511 and 3,799,235, commonly assigned to applicants assignee herein.
U.S. Pat. No. 3,683,511 discloses a method of removing volatile hydrocarbons from a solution or mixture of elastomer and volatile hydrocarbons, utilizing an extruder which involves the steps of working the elastomer in the extruder to increase its temperature; admixing an inert gas or water into the elastomer through an inlet in the extruder; allowing vapor to flash from the mixture thus formed through at least one vented section in the extruder and then discharging the elastomer from the end of the extruder at a lower content of volatile hydrocarbons. As further disclosed, the elastomer can be subjected to a plurality of such working, mixing, and flashing operations.
The method for drying solution-polymerized elastomers described in the aforementioned U.S. Pat. No. 3,683,511 represented a distinct advance in the art of drying polymers at the time of its discovery and still is advantageous in many respects. However, the method does have certain disadvantages. Thus, the method is not suitable for drying elastomer solutions or mixtures having volatile contents in excess of about 50 percent, since the viscosity of such solutions or mixtures is ordinarily too low to permit processing through an extruder. As will be evident, such a method would not be suitable for drying solution polymerized elastomers in which the solution often contains up to 85 percent by weight volatiles or more. An additional and significant disadvantage relates to so-called elastomer "fines". Thus, as the elastomer containing volatile hydrocarbons is passed through a zone of reduced pressure, the volatile hydrocarbon rapidly flashes to vapor, often with a violent and explosive force. Usually when this occurs, there is a marked tendency for at least a portion of the polymer to fragment, disintegrate or otherwise subdivide into very fine particles typically having a dimension of 0.5 millimeters or less. These fragmented materials are referred to in the art as "fines".
The "fines" present a number of problems. For example, they must be collected using hoods and other collection equipment to prevent undue contamination of the atmosphere. In addition to the loss of polymer caused by the fines, they also have a tendency to coat the surfaces of the hoods and collection equipment where, upon standing for prolonged periods of time, they degrade. More significantly, the fines often coat the vents of the extruder ultimately causing the vents to become plugged, thereby often resulting in a shut down of the extruder to enable clean up of the vents. As will be appreciated, such extruder shut downs result in a considerable loss of production.
U.S. Pat. No. 3,799,235 discloses a method of removing volatiles from a solution or mixture of elastomer and volatile material which involves releasing the mixture or solution from a zone maintained at a temperature above the normal boiling point of the volatile material and at an initial pressure at least sufficient to keep the volatile material in the liquid state through at least one orifice into one end of a conduit which is at a lower pressure than the initial pressure. Upon entering the conduit, some of the volatile material flashes into vapor and the remaining mixture or solution forms discrete elastomer crumbs. The vapor and elastomer crumbs then exit the conduit into an enclosure maintained at lower pressure following which the vapor is removed from the top of the enclosure and the elastomer crumbs are collected.
The method for drying elastomer solutions or mixtures described in the aforementioned U.S. Pat. No. 3,799,235 also represented a distinct advance in the art of drying polymers at the time of its discovery. However, the method has one distinct disadvantage. Thus, the method is limited in the amount of volatile materials which can be removed from elastomer solutions having a high content of volatiles. For example, as disclosed in the Examples of the patent, the finished elastomer product often contains as much as 15 percent by weight of volatiles or more. This is a considerable disadvantage since elastomers utilized in most commercial applications ordinarily should contain not more than 1.0 percent by weight volatiles and more desirably less than 0.3 percent by weight of volatiles.